1. Field of the Invention
The present invention relates to a fuel cell including at least one unit cell. The unit cell includes an electrolyte electrode assembly, and first and second separators for sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between a pair of separators. The membrane electrode assembly and the separators make up a unit cell for generating electricity.
In the unit cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Generally, several tens to hundreds of unit cells are stacked together to form a stack of the fuel cell. These unit cells need to be in alignment with each other accurately. In order to achieve the accurate positioning of the unit cells, typically, a knock pin is inserted in each of positioning holes formed in the unit cells. When a large number of the unit cells are stacked together, the operation of inserting the knock pins into the holes of the unit cells is laborious, and the fuel cell can not be assembled efficiently. Positional displacement between components may occur undesirably, and the desired sealing performance may not be achieved.
In an attempt to address the problem, techniques as disclosed in Japanese laid-open patent publication No. 2000-12067 and Japanese laid-open patent publication No. 7-29580 are proposed. Japanese laid-open patent publication No. 2000-12067 discloses a solid polymer electrolyte fuel cell 1 shown in FIG. 9. The fuel cell 1 includes a unit cell 2 and separators 3a, 3b for sandwiching the unit cell 2. The unit cell 2 includes a solid polymer electrolyte membrane 2a, an anode 2b provided on one surface of the solid polymer electrolyte membrane 2a, and a cathode 2c provided on the other surface of the solid polymer electrolyte membrane 2a. 
Holes 4 extend through the fuel cell 1 in a stacking direction of the fuel cell 1 for inserting holding pins 6. The separator 3b has openings 5 for inserting snap rings 7. The holding pin 6 has a snap ring attachment groove 6a. The holding pin 6 is inserted into the hole 4, the snap ring 7 is inserted into the opening 5, and the snap ring 7 is fitted to the snap ring attachment groove 6a. At one end of the holding pin 6, a chamfered tip 6b is formed. At the other end of the holding pin 6, a hole 6c for inserting the tip 6b of another holding pin 6 is formed.
As described above, in the system of the fuel cell 1, the holding pin 6 is inserted into the hole 4, and the snap ring 7 is inserted into the opening 5. The snap ring 7 is fitted to the snap ring attachment groove 6a for tightening the fuel cell 1.
Thus, the tip 6b of the holding pin 6 projecting from the outer surface of the separator 3b is fitted to the hole 6c of another holding pin. 6 which tightens another fuel cell 1. In this manner, the adjacent fuel cells 1 are stacked in alignment with each other.
Further, Japanese laid-open patent publication No. 7-29580 discloses a fuel cell shown in FIG. 10. The fuel cell includes a unit cell 8 of a rectangular parallelpiped. A pair of terminals 9a, 9b are provided on opposite sides of the unit cell 8. The terminal 9a is connected to an anode b, and the terminal 9b is connected to a cathode 2c. Further, holding members 9c, 9d are provided on opposite sides of the unit cell 8 in another direction. The components of the unit cell 8 are fastened together by the terminals 9a, 9b and the holding members 9c, 9d. 
According to the disclosure of Japanese laid-open patent publication No. 2000-12067, a plurality of the holding pins 6 need to be inserted into the holes 4 for each of the unit cells 2. Further, the snap rings 7 need to be fitted to the respective snap ring attachment grooves 6a of the holding pins 6. Thus, when a large number of unit cells 2 are stacked together, operation of assembling the holding pins 6 and the snap rings 7 is considerably laborious, and can not be performed efficiently.
According to the disclosure of Japanese laid-open patent publication No. 7-29580, each of the terminals 9a, 9b, and the holding members 9c, 9d has two holding portions HP extending from a base plate BP for holding the unit cell 8. The length of the holding portions HP is shorter than the length of the base plate BP. The unit cell 8 may not be held reliably between the holding portions HP. Thus, the terminals 9a, 9b, and the holding members 9a, 9b are disengaged easily from the unit cell 8.
The terminals 9a, 9b, and the holding members 9c, 9d cover the entire circumferential side surfaces of the unit cell 8. Therefore, the fuel cell is large and heavy. Further, the holding portions HP are overlapped with each other when the unit cells 8 are stacked. The dimension of the fuel cell in the stacking direction is large by the thickness of the holding portions HP stacked to cover the unit cells 8.